DMA repair and DNA damage checkpoints are important for tumor suppression, and are critical determinants of tumor resistance to radiation and chemotherapeutics. DNA double-strand breaks (DSBs) are produced by genotoxic chemicals and ionizing radiation, and arise spontaneously during DNA replication. DSBs are repaired by non-homologous end-joining (NHEJ) and homologous recombination (HR). DSBs occur in the context of chromatin, and chromatin alterations play important roles in the recruitment of DNA repair and checkpoint proteins to damage sites. The proposed studies are designed to determine the roles of chromatin modification and remodeling in protein recruitment to DSBs, and in the regulation of DSB repair outcome in the yeast Saccharomyces cerevisiae. Our central hypothesis is that DSB repair efficiency and outcome are regulated through the integrated actions of repair proteins, checkpoint proteins, and chromatin dynamics. We will test this hypothesis by pursuing three Specific Aims focused on (1) the roles of chromatin changes in recruitment of repair and checkpoint proteins to DSBs; (2) defining chromatin changes at a DSB site and a donor locus during HR; and (3) determining how chromatin regulates DSB repair. As with phosphorylation of histone H2A, we found that nucleosome displacement plays a role in the timely recruitment of repair proteins to DSBs. The roles of these and other chromatin alterations in protein recruitment to DSBs, regulation of DSB repair, and checkpoint activation will be investigated. These studies will clarify how chromatin alterations regulate cellular responses to DSBs and thereby determine the genetic consequences of DSB damage. The proposed studies are relevant because chromatin modifications, DSB repair, and checkpoint responses are conserved from yeast to man. The studies are significant because they will reveal new regulatory mechanisms in DSB repair, and thus identify new targets to exploit in cancer radio- and chemotherapy.